Originally a crankshaft drove the front end assembly drive (FEAD) system of an engine. The crankshaft was turned by the firing of pistons, which exerted a rhythmic torque on the crankshaft, rather than being continuous. This constant application and release of torque caused vacillations, which would stress the crankshaft to the point of failure. Stated another way the crankshaft is like a plain torsion-bar, which has a mass and a torsional spring rate, that causes the crankshaft to have its own torsional resonant frequency. The torque peaks and valleys plus the inertia load from the acceleration of the reciprocating components causes the crankshaft itself to deflect (rotationally) forward and backward while it is operating. When those pulses are near the crankshaft resonant frequency, they would cause the crank to vibrate uncontrollably and eventually break. Accordingly, a torsional vibration damper (sometimes referred to as a crankshaft damper) is mounted on the crankshaft to solve this problem by counteracting torque to the crank, negating the torque twisting amplitude placed upon the crankshaft by periodic firing impulses, and to transfer rotational motion into the FEAD system, typically by driving an endless power transmission belt.
While existing torsional vibration dampers have been effective to extend the life of the crankshaft and to drive the FEAD system, changes in vehicle engine operation, such as the introduction of start-stop systems to conserve fuel consumption, add complexities to the system that the existing torsional vibration dampers are not designed to address. For instance, the start-stop system introduces impact forces due to belt starts that introduce potential slip in the elastomer-metal interface in traditional torsion vibration dampers. Another concern is maintaining good axial and radial run-outs between the metallic components.
Also, starting and stopping causes the crankshaft to experience rigid body modes of vibration, which are transmitted through the torsion vibration damper to the FEAD. During starting and stopping conditions, the crankshaft rotates at lower rotational speeds (rpms) that have frequencies in the range of the natural frequency of the crankshaft. When the rotational speed of the crankshaft approaches the natural frequency of vibration of the crankshaft itself, the rotational frequency amplifies/excites the natural frequency of vibration of the crankshaft, which results in a resonance condition. This resonance causes rigid body mode vibrations. In the absence of an isolator, these rigid body mode vibrations are transmitted unnecessarily to the FEAD, which can cause wear and damage to the components of the FEAD.
Some torsional vibration dampers also include an isolator system to reduce transmission of these rigid body modes of vibration to the FEAD. Some of these isolator systems use a rubber spring for isolation as well as for the vibration damper. Typically, these isolators are mold-bonded to another component of the torsional vibration damper and/or involve several moving components. Mold-bonding adds expense to the manufacturing process by requiring special equipment and time to accomplish the molding process. Elimination of this step or requirement would be beneficial. Additionally, minimizing the number of moving components in and reducing the overall cost of torsional vibration damper-isolators is beneficial. Accordingly, improved designs for torsional vibration dampers having isolators are needed.